SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Quarterly Financial Information Disclosure [Abstract]
Operating revenues	$ 479,081	$ 434,942	$ 866,255	$ 811,767	$ 647,326	$ 543,435	$ 1,019,043	$ 705,305	$ 2,592,045	$ 2,915,109	$ 2,268,617
Operating income (loss)	(7,790)	(4,019)	77,001	88,743	(18,343)	(36,715)	178,744	76,196	153,935	199,882	171,224
Net income (loss)	$ 18,086	$ (8,402)	$ 73,573	$ 86,248	$ (16,255)	$ (14,274)	$ 140,266	$ 123,699	$ 169,505	$ 233,436	$ 132,065
Earnings (loss) per share
Basic (in dollars per share)	$ 0.20	$ (0.09)	$ 0.83	$ 0.97	$ (0.18)	$ (0.16)	$ 1.60	$ 1.42	$ 1.90	$ 2.66	$ 1.53
Diluted (in dollars per share)	$ 0.20	$ (0.09)	$ 0.82	$ 0.97	$ (0.18)	$ (0.16)	$ 1.59	$ 1.42	$ 1.89	$ 2.64	$ 1.52